The present invention relates to a method for codepositing water-insoluble materials such as inorganic or organic particles or short fibers in metal deposits.
It was proposed in the past to codeposit water-insoluble materials such as organic or inorganic particles in metal deposits in order to impart to them certain properties such as water resistance, lubricity, and adhesion.
In such codeposition methods, particularly, when reduced to commercial practice, it is desirable to ensure stable codeposition of water-insoluble materials. It is also desirable to codeposit water insoluble materials uniformly in a greater proportion with metal deposits. Although a proposal to this end has been made to use a special surface-active agent to suspend water-insoluble material in a plating solution, it is still necessary to ensure a more stable codeposition of water-insoluble material in metal deposits.
After extensive research, it has been found that a water-insoluble material can be codeposited with a metal in a very stable manner and in a greater proportion by carrying out deposition while circulating and fluidizing a plating solution having the metal ion dissolved therein and the water-insoluble material suspended therein by means of a pump. More specifically, the plating solution is circulated by the pump by pumping at least one third, preferably at least one-half of the volume of the plating solution per minute. A sparger pipe having a plurality of holes formed in its lower side is connected to the pump and disposed at a lower level in a tank. The pumped volume of the plating solution is injected downward into the solution through the sparger holes, and the plating solution is thus circulated and fluidized without entraining air bubbles.
In the prior art methods of codeposition, plating solutions having water-insoluble materials suspended therein are agitated by a variety of techniques including air agitation, mechanical agitation using, for example, an impeller agitator, and bath fluidization, for example, pumping circulation. Alternatively, workpieces themselves may be swung or vibrated in a plating solution. The inventors have found that agitation of a plating solution having water-insoluble materials suspended therein has a significant influence on the stable codeposition of the water-insoluble materials in the metal deposits. According to the inventors' experience, air agitation does not work well, particularly when a surface-active agent is added to a plating solution in order to more stably disperse water-insoluble materials in the solution. Codeposition becomes less stable because air bubbling causes foaming in the presence of a surface-active agent and such foams envelop the water-insoluble materials to reduce the quantity of the water-insoluble materials codeposited, which varies with different batches. The use of an impeller agitator often results in a unevenly fluidized plating bath. This in turn, results in a local variation in the quantity of the water-insoluble materials codeposited on the workpiece when a workpiece to be plated is of relatively large size, or a variation in the quantity of the water-insoluble materials codeposited among the workpieces when materials are codeposited concurrently on a plurality of workpieces. Further, the quantity of the water-insoluble materials codeposited substantially varies with the position of an impeller agitator relative to the position, orientation, and other dimensional factors of a workpiece to be plated, which causes less stable codeposition. Additionally, the quantity of the water-insoluble materials codeposited is relatively small. Further, the use of a pump for liquid circulation also suffers from a problem similar to the impeller agitator because the pump discharge opening must be critically positioned relative to the position, orientation, and dimensional factors of a workpiece. The technique of swinging or vibrating a workpiece in a plating bath also has a problem in decreasing the quantity of the water-insoluble materials codeposited. Under the circumstances, the inventors have studied a variety of agitation techniques to find that when codeposition is carried out from a plating solution having a metal ion dissolved and a water-insoluble material suspended therein, by pumping at least one third of the volume of the solution per minute by means of a pump without entraining air bubbles, and injecting the thus pumped volume downward into the solution through a plurality of holes in the lower side of a sparger pipe connected to the pump and located at a lower level in a tank, thereby circulating and fluidizing the plating solution, then highly stable codeposition is carried out in a reproduceable manner, and the quantity of the water-insoluble material codeposited is increased with a minimal variation. The present invention is based on this finding.